A hydraulic driving system that performs load-sensing control to control a capacity of a hydraulic pump such that a differential pressure between a delivery pressure of the hydraulic pump and the highest load pressure of a plurality of actuators is maintained at a target differential pressure has traditionally been used in construction machines such as hydraulic excavators. Patent Document 1 describes an example of such a hydraulic driving system.
The hydraulic driving system described in Patent Document 1 includes a differential pressure reducing valve configured to output, as an absolute pressure, the differential pressure between the delivery pressure of the hydraulic pump and the highest load pressure of the plurality of actuators, and the absolute pressure is introduced as a feedback load-sensing (LS) differential pressure into an LS control valve of a pump regulator, and further an absolute pressure varied according to revolution speed of an engine is introduced into the LS control valve as a target LS differential pressure to perform load-sensing control. In addition, the absolute pressure output from the differential pressure reducing valve (the differential pressure between the delivery pressure of the hydraulic pump and the highest load pressure) is introduced into a plurality of pressure compensating valves as a target compensation differential pressure to control the differential pressures across flow control valves.
By introducing the differential pressure between the delivery pressure of the hydraulic pump and the highest load pressure into the plurality of pressure compensating valves as the target compensation differential pressure and controlling the differential pressures across the flow control valves in this way, when two or more actuators are simultaneously operated, if there occurs saturation in which a flow rate of the hydraulic fluid delivered from the hydraulic pump is less than those demanded by the flow control valves, the differential pressure between the delivery pressure of the hydraulic pump and the highest load pressure decreases in accordance with the degree of saturation, which in turn reduces the target compensation differential pressure across the particular pressure compensating valve and hence the differential pressure across the particular flow control valve. The flow rate of the hydraulic fluid delivered from the hydraulic pump, therefore, can be redistributed according to a ratio of the flow rates demanded by the flow control valves, and as a result, appropriate operability can be obtained during such combined operation.
Further, by performing load-sensing control such that the absolute pressure, which is variable in accordance with the revolution speed of the engine, is used as the target LS differential pressure and introduced into the LS control valve, when the revolution speed of the engine is reduced from its rating, the target LS differential pressure correspondingly decreases. Thus, the flow rate of the hydraulic fluid supplied from the hydraulic pump to the actuators also decreases, which enables fine operability to improve.
In the hydraulic driving system that introduces the differential pressure between the delivery pressure of the hydraulic pump and the highest load pressure into the pressure compensating valves as the target compensation differential pressure, when two or more actuators are operated at the same time, in cases where one of the actuators is of a cylinder type and this actuator reaches a stroke end, the differential pressure between the delivery pressure of the hydraulic pump and the highest load pressure becomes zero (0) and hence the target compensation differential pressure also becomes 0, which fully closes the pressure compensating valves and stop the other actuator(s).
The hydraulic driving system described in Patent Document 1 employs a measure for preventing such a stoppage of an actuator. More specifically, the system further includes, in a highest load pressure line, a signal pressure variable relief valve that renders a set pressure of the valve changeable according to the particular target compensation differential pressure. When a specific actuator reaches a stroke end and the delivery pressure of the hydraulic pump increases to a set pressure of a main relief valve, the system activates the signal pressure variable relief valve to limit the maximum pressure of the highest load pressure to a pressure lower than the set pressure of the main relief valve. Accordingly, even after the specific actuator has reached its stroke end, the differential pressure between the delivery pressure of the hydraulic pump and the highest load pressure does not become 0, which prevents the pressure compensating valves from fully closing, prevent the other actuator(s) from stopping, and maintain the appropriate operability during combined operation.
On the other hand, so-called boost circuits are known. These circuits are designed such that only when a specific actuator is operated, the circuit increases the set pressure of the main relief valve by a predetermined value from a first value to a second value and increases the maximum delivery pressure of the hydraulic pump. Patent Document 2 describes an example of such boost circuits.
The traveling excavation machine, such as a hydraulic excavator, that is described in Patent Document 2 includes a main relief valve configured as a variable relief valve so as to increase a pressure setting of the main relief valve from a first value to a second value only when an operating pilot pressure for a track operating device is introduced into the main relief valve and a control lever of the track operating device is operated. This configuration of the machine ensures generation of the output torque required of track motors during track operation, and improves traveling performance of the machine.